Optical or photoresponsive sensors are often used in applications to determine the presence of a copy or document sheet passing through a certain point by providing a suitable signal in response to the sheet. Typically, the optical sensor includes a light source whose light beam is directed at the position at which the sheet is to be sensed. A light sensitive transducer, for example a phototransistor or photodiode, is mounted in an aligned relationship with the light source, to receive either direct or reflected light from the light source.
A recurring problem in reproduction machines is the contamination of optical sensors, particularly those in the paper path, by airborne toner particles, paper fibers, carrier particles, and other contaminants. These contaminants generally cause failure by coating the optical elements, thereby greatly reducing the illumination level detected at the sensor. An additional problem is the potential for misalignment of the sensor which may also result in reduced illumination of the sensor, potentially resulting in false or erratic detection when the sensor is vibrated or otherwise perturbed, even when a copy sheet is not present.
One solution to these problems is to schedule frequent preventative maintenance periods to clean the sensors and to test the level of performance. However, this can be very costly in terms of personnel and increased down time of the machine.
Yet another potential problem for optical sensors is the degradation of the sensors through aging of the light source, producing a corresponding decrease in light output in the sensing region.
Heretofore, sensor degradation problems have been recognized and methods for correcting or allowing for the degraded performance have been proposed, some of which are found in the following disclosures which may be relevant:
U.S. Pat. No. 3,789,215, Patentee: Penny, Issued: Jan. 29, 1974.
U.S. Pat. No. 3,836,248, Patentee: John, Jr., Issued: Sep. 17, 1974.
U.S. Pat. No. 4,046,469, Patentee: Frank et al., Issued: Sep. 6, 1977.
U.S. Pat. No. 4,097,731, Patentee: Krause et al., Issued: Jun. 27, 1978.
U.S. Pat. No. 4,097,732, Patentee: Krause et al., Issued Jun. 27, 1978.
U.S. Pat. No. 4,310,153, Patentee: Kikucki, Issued: Jan. 12, 1982.
U.S. Pat. No. 4,365,151, Patentee: Fasig et al., Issued: Dec. 21, 1982.
U.S. Pat. No. 4,520,270, Patentee: Enter et al., Issued: May 28, 1985.
U.S. Pat. No. 4,577,096, Patentee: Beery et al., Issued: Mar. 18, 1986.
U.S. Pat. No. 4,670,647, Patentee: Hubble, III et al., Issued: Jun. 2, 1987.
U.S. Pat. No. 4,740,686, Patentee: Nusser, Issued: Apr. 26, 1988.
U.S. Pat. No. 4,818,132, Patentee: Brull et al., Issued: Apr. 4, 1989.
U.S. Pat. No. 5,008,532, Patentee: Ono et al., Issued: Apr. 16, 1991.
Document Sensor, Xerox Disclosure Journal, Vol. 7, No. 1, January/February 1982.
Paper Edge Sensor, IBM Technical Disclosure Bulletin, Vol. 23, No. 7A, December 1980.
Optical Sensor Output Amplifier, IBM Technical Disclosure Bulletin, Vol. 24, No. 3, August 1981.
The relevant portions of the foregoing patents and disclosures may be briefly summarized as follows:
U.S. Pat. No. 3,789,215 discloses a photosensitive circuit used for the detection of document sheets passing a counting station. The document detecting circuit further includes means for adjusting the document detecting threshold, relative to a peak output signal, in order to compensate for multiple documents or long term variation in the light source.
U.S. Pat. No. 3,836,248 teaches an optical sensor employing a plurality of sensors, whereby detection is determined using signals generated by a bridge circuit to balance the output of a sensor which always has light impinging on it and a second sensor having light transmitted through a document sensing region. The bridge circuit generates a signal when it becomes unbalanced due to the presence of a document in the document sensing region, thereby enabling the detection of translucent paper stock.
U.S. Pat. No. 4,046,469 describes a photocopy machine having a paper cut-off sensor. The sensor is a reflective-type optical sensor utilizing an infrared light source modulated at a predetermined frequency. False detection of the sheet is avoided by sensing the modulation of the emitted light.
U.S. Pat. Nos. 4,097,731 4,097,732 teach a sensor for regulating the intensity of the sensor light source to compensate for extraneous factors in the operating environment such as dust accumulation, component aging and misalignment. However, this type of compensation, adjusting the power output of the lamp is often relatively expensive, due to the feedback circuitry required, and generally provides only a limited degree of adjustment.
U.S. Pat. No. 4,310,153 discloses plural optical sensors in an electrostatographic system whereby the sensors are systematically activated, in response to a timing signal, to determine the presence of a sheet in accordance with the anticipated travel of the sheet.
U.S. Pat. No. 4,365,151 describes a sensor for a document processor having a reflective type sensor. The path of the light reflected from a cylindrical surface, versus the light reflected from the paper surface is sufficiently different to cause the reflected light to miss the detector when a paper sheet is present, thus generating a signal sufficient to indicate the presence of the paper.
U.S. Pat. No. 4,520,270 teaches a circuit for self-adjusting the sensitivity of a sensor. Calibration of each sensor is conducted independently by varying the voltage supplied to a comparator. Specifically, by applying the sensor output to one input of the comparator, and a reference voltage, generated as an output of a digital-to-analog converter, to the other input, the sensor open and sensor closed potentials can be determined.
U.S. Pat. No. 4,577,096, assigned to Xerox Corporation, discloses a photodetector employing a resistor-capacitor (RC) network where a feedback loop provides an increase in power whenever the transmissibility of the sensor changes. In addition, an operational amplifier may be set to provide a signal when an excessive amount of power is needed to drive the lamp at a level detectable by the phototransistor.
U.S. Pat. No. 4,670,647, assigned to Xerox Corporation and hereby incorporated by reference for its teachings, describes a dirt insensitive optical paper path sensor. The self-adjusting document sensor compensates for any degradation in the sensor system by feeding the output of the detector into an amplifier having a variable gain. The amplifier gain is controlled by a feedback signal generated as a function of the amount of deviation from a reference signal.
U.S. Pat. No. 4,740,686 teaches an improved sensor circuit suitable for reducing the current required to operate the sensor in a standby condition.
U.S. Pat. No. 4,818,132 describes a reflective-type optical sensor which bridges a paper path to detect the presence of a slip sheet. A non-reflecting brush, opposed to the sensor face, serves the dual purpose of cleaning the sensor face and covering the face to prevent false detection when the sheet is not present.
U.S. Pat. No. 5,008,532 discloses the use of a programmable attenuator in the feedback circuit which conditions the output signal of the photoelectric sensor. Also included in the feedback circuit are a pair of switchable resistors (R.sub.11 and R.sub.12), used to control the feedback circuit, thereby providing two ranges for the attenuator.
The Xerox Corporation and IBM Corp. disclosures describe various circuits used to implement paper and edge sensing with optical sensors.
In accordance with the present invention, there is provided an an optical sensor having an alterable sensitivity. The sensor has a light source for emitting light and a detector, disposed in aligned relationship with the light source, for producing a first output signal level in response to light having an illumination intensity greater than or equal to a threshold intensity, and a second output signal level in response to light having an illumination intensity less than a threshold intensity. The sensor also includes means, associated with the light source, for selectively altering the illumination intensity of the light emitted by the sensor. The illumination intensity is alterable between a first intensity which is greater than the detector threshold intensity and a second intensity which is greater than the detector threshold intensity and less than the first intensity, thereby altering the sensitivity of the sensor as a function of the selected illumination intensity.
In accordance with another aspect of the present invention, there is provided a sheet transport apparatus having sheet transport rolls, sheet guides, and an optical sheet sensor. The optical sheet sensor includes a light source and a photodetector, disposed in aligned relationship with the light source, for producing a first signal level indicating detection of light emitted by the light source when the sheet is not present and a second signal level indicating no detection of the emitted light when the sheet is present. The apparatus also includes means, associated with the light source, for regulating the intensity of the light impinging upon the photodetector as a function of a predetermined sheet transmissivity.
In accordance with yet another aspect of the present invention, there is provided an apparatus, disposed within an electrophotographic printing system having a document transporting mechanism associated therewith, to recognize when an optical document sensor within the document transporting mechanism is operating in a degraded condition. The apparatus includes an optical document sensor having a variable intensity light source and a photodetector disposed in aligned relationship with one another, where the intensity of the light varies as a function of an electrical current which powers the light source and where the photodetector is responsive to the light intensity produced by the light source. The apparatus also includes means, responsive to a mode selection signal, for switching the amount of current supplied to the light source between a first current level and a second current level lower in magnitude than the first current level, and control means, connected to the switching means. The control means comprises means for producing the mode selection signal to cause the current switching means to produce the lower current level, and means, connected to the photodetector, for monitoring an output signal produced by the photodetector during the lower current mode to determine if the output signal is less than a predetermined minimum level, and if so, indicating that the document sensor is operating in a degraded condition.
In accordance with another aspect of the present invention, there is provided a method of altering the sensitivity of an optical sheet sensor in order to sense a translucent sheet present within a sheet sensing region. The sensor has a light source disposed in aligned relationship with a photodetector to form the sheet sensing region therebetween. The method comprises the steps of determining that a translucent sheet is to be sensed by the optical sheet sensor, and decreasing, in response to determining the presence of the translucent sheet, the intensity of the light transferred from the light source to the photodetector and thereby increasing the sensitivity of the optical sheet sensor.
In accordance with yet another aspect of the present invention, there is provided a method of determining when an optical sensor, having a light source and a photodetector disposed in an aligned, light receiving relationship with one another, is operating in a degraded condition. The method includes the steps of: 1) reducing the intensity of the light received by the photodetector to a low illumination intensity below the normal operating intensity yet above the photodetector threshold intensity; 2) testing an output signal produced by the photodetector to determine if the photodetector is responsive to light emitted at the low illumination intensity; and if not, 3) identifying the sensor as operating in a degraded condition.